Upgrade from SP2007 to SP2010

I have decided to start an online project where I can update online community with real world challenges that I am going to meet met during the Upgrade SharePoint 2007. The project starts July 1st and is supposed to end in December 2011. Actual end date was the beginning of May 2012.

It took 11 months to complete the project. The delay happened for several reasons:
 - we waited for new machines to build a new farm (as we adopted Hybrid approach of migration);
 - then we waited for testers to be available to  run their validation,
 - and we waited a little bit more to get developer resources to work on compatibility issues.
Since our SharePoint Applications were highly customized, we needed to address some code incompatibilities (one of them is BDC API). It took 2 months of fixes to get a compatible version of our wsp files.

Here are related posts:

The farm configuration that is about to be upgraded:
"SharePoint 2007 to 2010 Upgrade" online project (part 1) : "The start"

First upgrade trial - reflection on the custom master page and a new feature - Developer dashboard:
"SharePoint 2007 to 2010 Upgrade" online project: Developer dashboard on the custom page\master.page 

We started with a plan to run in-place upgrade, which is riskier but is considered faster.
Then we choose to settle down with Hybrid upgrade. It gave us low-risk, less-stressful upgrade.

"SharePoint 2007 to 2010 Upgrade" online project (part 2): Joel Oleson's Upgrade to SharePoint 2010 Best Practices 

Cliff notes from Microsoft SharePoint 2010 Administrator's Companion  "Upgrading to Sharepoint 2010":
"SharePoint 2007 to 2010 Upgrade" online project (part 3): "Theory" 

The Hardware and Software requirements for SharePoint 2010:
"SharePoint 2007 to 2010 Upgrade" online project (part 4): "Preparing to install SharePoint 2010. Prerequisites"

Before to roll SharePoint 2010 I want to make sure that my SharePoint 2007 is fully backup and can be restored in case of Upgrade failure:
"SharePoint 2007 to 2010 Upgrade" online project (part 5) : Backup strategy

Ask the client about the visual upgrade. If they opt to it, make sure the client is aware of UI changes and will take care of the end-user training.
In our case, the end-users were taken by surprise. First couple weeks we have got a constant feedback that they  "miss" the features that they are used to have.

 Everything you need to know about "Upgrade to SharePoint 2010" on   MSDN: Upgrading to SharePoint Server 2010

Because we have BDC files, we need to take care of their upgrade. We could keep old format BDC, but we prefered to have them upgraded to make the support easier.

That's something you want to include in your estimates.

"SharePoint 2007 to 2010 Upgrade" online project (part 6) : Database attach upgrade. BDC to BCS conversion

"SharePoint 2007 to 2010 Upgrade" online project (part 7) : In-place upgrade BDC failed: the bdc service application is not accessible There are no addresses available for this application

Plan and test configuration of the service applications
Especially, User Profile Synchronization service. 

"SharePoint 2007 to 2010 Upgrade" online project (part 8) : How to start User Profile Synchronization service

And final series on the Upgrade:

What service accounts you should have for SharePoint 2010: Service Account Suggestions for SharePoint 2010 by Todd Klindt

"SharePoint 2007 to 2010 Upgrade" online project (part 9) : SharePoint service accounts

Plan upgrade per env (worked well for me to go thru several environment before to upgrade Prod)
Plan upgrade actions
Keep the old prod for reference for some time.

"SharePoint 2007 to 2010 Upgrade" online project (part 10) : Upgrade strategy plan

SP2010 was a good and rewarding experience for me. I remember more organizational challenges than technical.
I wish you the same. You may want to consider to browse SharePoint best practices to make sure that your upgrade will be smooth on the technical side.